The present invention involves recombinant equine herpesviruses useful in the preparation of vaccines to protect horses from various species of naturally-occurring infectious equine herpesvirus. The equine herpesvirus is a member of the family herpesviridae, which are commonly known as the herpesviruses.
Generally, herpesviruses contain 100,000 to 200,000 base pairs of DNA as their genetic material, and several areas of the genomes of various members have been identified that are not essential for the replication of virus in vitro in cell culture. Modifications of these regions of the DNA have been known to lower the pathogenicity of the virus, i.e. to attenuate the virus when it infects an animal species. For example, inactivation of the thymidine kinase gene of either human herpes simplex virus (29) or pseudorabies virus of swine (38) renders these herpesviruses less pathogenic.
Removal of specific regions of the repeat region of a human herpes simplex virus have been shown to render the virus less pathogenic (32, 39). Furthermore, a repeat region has been identified in Marek's disease virus that is associated with viral oncogenicity (13). A region in herpesvirus saimiri has similarly been correlated with oncogenicity (21). Removal of a specific region of the repeat region renders pseudorabies virus less pathogenic (U.S. Pat. No. 4,877,737). A region in pseudorabies virus has been shown to be deleted in naturally-occurring vaccine strains (22). These deletions are at least in part responsible for the lack of pathogenicity of these strains.
It is generally agreed that herpesviruses contain non-essential regions of DNA in various parts of the genome, and that modification of these regions can attenuate the virus, leading to a non-pathogenic strain from which a vaccine may be derived. The degree of attenuation of the virus is important to the utility of the virus as a vaccine. Deletions which cause too much attenuation of the virus will result in a vaccine that fails to elicit an adequate immune response. Although several examples of attenuating deletions are known, the appropriate combination of deletions for any herpesvirus is not readily apparent.
Major economic losses to the equine industry result from infection by two species of equine herpesvirus (17). These two equine herpesvirus species, currently identified in the literature as EHV-1 and EHV-4, belong to the herpesvirus sub-family alpha-herpesvirus and are characterized by a class D genome (33). Formerly, both species were identified as EHV-1 and further differentiated as EHV-1 subtype 1 (EHV-1) and EHV-1 subtype 2 (EHV-4) respectively. EHV-1 is the primary cause of abortion in pregnant mares and EHV-4 is the primary cause of respiratory disease in foals and yearlings. Currently available products are not designed to address both disease syndromes, with the result that these products are marginally effective.
EHV-1 and EHV-4 have been analyzed at the molecular level. Restriction maps of the genomes of EHV-1 and EHV-4 have been reported (42 and 8).
Although several of the herpesviruses have been genetically engineered, no examples of recombinant EHV have been reported.
EHV can become latent in healthy animals which makes them potential carriers of the virus. For-this reason, it is clearly advantageous to be able to distinguish animals vaccinated with non-virulent virus from animals infected with disease-causing wild-type or naturally-occurring virus. The development of differential vaccines and companion diagnostic tests has proven valuable in the management of pseudorabies disease (Federal Register, Vol. 55, No. 90, pp. 19245-19253). A similar differential marker vaccine would be of great value in the management of EHV caused disease.
The present invention provides a method of producing a fetal-safe, live recombinant EHV virus which comprises treating viral DNA from a naturally-occurring live EHV so as to delete from such viral DNA, DNA corresponding to the US2 gene of the naturally-occurring EHV. The present invention also provides viruses in which (a) DNA corresponding to the US2 gene has been deleted, and (b) DNA encoding gpG, gpE, and/or TK has been altered or deleted. Such viruses are useful for the creation of vaccines which require diagnostic markers and safety in pregnant animals.
The ability to engineer DNA viruses with large genomes, such as vaccinia virus and the herpesviruses, has led to the finding that these recombinant viruses can be used as vectors to deliver vaccine antigens and therapeutic agents for animals. The herpesviruses are attractive candidates for development as vectors because their host range is primarily limited to a single target species (16) and they have the capacity for establishing latent infection (7) that could provide for stable in vivo expression of a foreign gene. Although several herpesvirus species have been engineered to express foreign gene products, recombinant equine herpesviruses expressing foreign gene products have not been constructed. The equine herpesviruses described above may be used as vectors for the delivery of vaccine antigens from microorganisms causing important equine diseases. Such multivalent recombinant viruses would protect against EHV as well as other diseases. Similarly the equine herpesviruses may be used as vectors for the delivery of therapeutic agents. The therapeutic agent that is delivered by a viral vector of the present invention must be a biological molecule that is a by-product of equine herpesvirus replication. This limits the therapeutic agent in the first analysis to either DNA, RNA or protein. There are examples of therapeutic agents from each of these classes of compounds in the form of anti-sense DNA, anti-sense RNA (19), ribozymes (41), suppressor tRNAs (3), interferon-inducing double stranded RNA and numerous examples of protein therapeutics, from hormones, e.g., insulin, to lymphokines, e.g., interferons and interleukins, to natural opiates. The discovery of these therapeutic agents and the elucidation of their structure and function does not necessarily allow one to use them in a viral vector delivery system, however, because of the experimentation necessary to determine whether an appropriate insertion site exists.